1. In large oven-proof skillet, saute onion in hot butter for 4 to 5 minutes or until onion is transparent.
2. Stir in flour, salt & pepper, and evaporated milk.
3. Cook this mixture over medium-low heat, stirring constantly, until it starts bubbling.
4. Add cheese and stir until melted.
5. Mix together in the skillet (or in a baking dish if your skillet isn't oven-proof) the rice, paprika, oregano, and tomatoes.
6. Add the tuna on top and toss gently, so the tuna doesn't break up too much, just a little.
7. Bake uncovered at 375 degrees for 25 to 30 minutes.

Story

During the last 10 minutes of cooking time, you can sprinkle some buttered bread crumbs on top if you like a crunchy top layer. This casserole is very creamy, thanks to the evaporated milk. Kids seem to like it for that reason, and also because of the cheese, of course. Enjoy!